In a variety of transmission systems, a transmission signal becomes distorted due to dispersion, loss and the like caused by the transmission medium. An equalization filter is known as a technology for electrically compensating this transmission signal for distortion. Whereas there are a plurality of types of equalization filters, a weighted delay equalization filter is often used.
The weighted delay equalization filter is a feed forward type equalization filter, and is also referred to as a transversal filter, an FIR (Finite Impulse Response) digital filter, or a feed forward equalizer. Also, a weighted delay equalization filter is described, for example, in Non-Patent Document 1 (A. Borjak, et al., “High-Speed Generalized Distributed-Amplifier-Based Transversal-Filter Topology for Optical Communication Systems,” IEEE Trans. Microwave Theory Tech., vol. 45, No. 8, pp. 1453-1457). In the following, the weighted delay equalization filter is called the “transversal filter.”
FIG. 1 is a schematic diagram showing the configuration of a conventional transversal filter. In FIG. 1, the transversal filter comprises a plurality of delay elements, a plurality of weighting circuits, and an adder.
Respective delay elements are connected in cascade, and sequentially delay a signal applied to the transversal filter (hereinafter referred to as the “input signal”). Each of the weighting circuits multiplies one of the input signal or an output signal of each delay element by a weighting value (filter coefficient). The adder adds respective output signals of the respective weighting circuits.
In this regard, when a transmission signal is an electric signal, the transmission signal is applied to a weighted delay equalization filter. On the other hand, when a transmission signal is an optical signal, the optical signal is converted to an electric signal by a photo-diode or the like, and the converted electric signal is applied to an equalization filter.
An input signal can be compensated for distortion by using such a transversal filter.
The configuration of a transversal filter is described, for example, in Non-Patent Document 2 (Differential 4-tap and 7-tap Transverse Filters in SiGe for 10 Gb/s Multimode Fiber Optic Equalization in Preprint paper 10.4 of International Solid-State Circ[UI]t Conference (ISSCC) 2003). FIG. 2 is a circuit diagram showing the configuration of a transversal filter described in Non-Patent Document 2.
In FIG. 2, the transversal filter has the configuration of a distributed amplifier circuit, and comprises filter input terminal 801, filter output terminal 802, delay devices 803 and 804, weighting circuit 805, 50Ω load resistances 806, 807, input load resistance 808, and output load resistance 809.
Delay device 803 comprises a plurality of delay elements each formed of a 50Ω matched transmission path connected to filter input terminal 801 in cascade. Delay device 804 comprises a plurality of delay elements each formed of a 50Ω matched transmission path connected to filter output terminal 802 in cascade.
Weighting circuit 805 is formed of an amplifier with a plurality of gain adjustment terminals, and comprises a plurality of weighting circuits which are disposed at locations corresponding to respective amplification stages of a distributed amplifier circuit. A filter coefficient varies in response to adjustments to respective amplification amounts of the plurality of amplifiers. When signal distortion during transmission varies over time, the transversal filter compensates the transmission signal for distortion by varying the filter coefficient over time.
50Ω load resistance 807 is connected to delay device 804 in cascade to form an adder.
Non-Patent Document 1: A. Borjak, et al., “High-Speed Generalized Distributed-Amplifier-Based Transversal-Filter Topology for Optical Communication Systems,” IEEE Trans. Microwave Theory Tech., vol. 45, No. 8, pp. 1453-1457
Non-Patent Document 2: Differential 4-tap and 7-tap Transverse Filters in SiGe for 10 Gb/s Multimode Fiber Optic Equalization in Preprint paper 10.4 of International Solid-State Circ[UI]t Conference (ISSCC) 2003